Superalloy components of gas turbine engines, e.g. turbine blades, turbine vanes, combustion chambers, are operated in a wide range of temperatures, for example from 600° C. to 1200° C. The temperature of operation of the superalloy component depends upon the position of the component within the gas turbine engine. During the selection of a superalloy for a component, or of a coating alloy for a component, it is essential to consider the environmental degradation of the superalloy, or the coating alloy, over all of the operating conditions that the superalloy component or the coating alloy will experience.
The main environmental degradation concerns for superalloy components, or coating alloys, are hot corrosion and oxidation at high temperatures. Hot corrosion is most active at temperatures below about 950° C. Hot corrosion occurs whenever salt, ash or other airborne contaminant deposit accumulates on the surface of the superalloy component, or coating alloy, and hence alter the surface reactions. The severity of hot corrosion may vary substantially and depends upon the content of impurity in the intake air and the content of impurity in the fuel. There are two types of hot corrosion, Type I and Type II. Type I and Type II hot corrosion behaviour of superalloy components, or coating alloys, is tested using either burner rig testing or laboratory furnace testing. However, at temperatures above 1000° C. oxidation is the dominant environmental degradation concern for superalloy components and coating alloys and oxidation is the life limiting factor for many superalloys and coating alloys.
Conventionally burner rig testing has been used to test superalloy components, or coating alloys, to assess their oxidation behaviour above temperatures of 1000° C. The burner rig test is capable of reproducing degradation mechanisms similar to those of an engine run blade/vane or an engine run blade/vane and coating.
However, burner rig testing suffers from several problems. It is difficult to control the temperature within a specified margin of less than 10° C. and especially so at temperatures above 1100° C. It is difficult to control contaminants deliberately introduced during a burner rig test. Burner rig testing has high operating costs due to the costs of the fuel and burner rig testing produces pollution. Burner rig testing is difficult with small quantities of superalloy or small quantities of coating alloy, e.g. small superalloy test pieces or small coating alloy samples on a superalloy test pieces. Burner rig testing has poor reproducibility from one burner rig to another resulting in substantial variation in lifing data generated by burner rigs at different sites.
More recently cyclic oxidation testing in air has been used to test superalloy components, or coating alloys, to assess their behaviour above temperatures of 1000° C.
Cyclic oxidation testing in air has many advantages compared to burner rig testing. There is good control of temperature within a specified margin of less than 10° C. Cyclic oxidation testing in air may be used with small quantities of superalloy or small quantities of coating alloy, e.g. small superalloy test pieces or small coating alloy samples on a superalloy test pieces. Cyclic oxidation testing in air has good reproducibility from one cyclic oxidation testing rig to another. Cyclic oxidation testing has lower operating costs and cyclic oxidation testing produces significantly less pollution.
However, cyclic oxidation testing cannot generate the degradation mechanism detected on superalloy gas turbine engine components, or coating alloys on gas turbine engine components, which have operated in a gas turbine engine because it has not taken into account the effects of contamination during the operation of the gas turbine engine. Cyclic oxidation testing has to be run for a long period of time, this may be 2000 to 4000 hours at 1100° C., to achieve any significant loss of material. The long operating periods of the cyclic oxidation testing increases the costs of the testing and impedes the development of new superalloys for components and the development of new coating alloys.
Therefore the present disclosure seeks to provide a novel method of testing the oxidation resistance of a superalloy, or a coating alloy, which reduces or overcomes the above mentioned problem.